The present invention relates to a secondary oxygen purifier for a molecular sieve oxygen concentrator.
Molecular sieve oxygen concentrators have attracted considerable attention recently because they are capable of producing high purity oxygen (about 95%) in a simple, cost-effective manner. Further, this oxygen has been found acceptable as a breathing gas for patients requiring oxygen therapy and for aircrew hypoxia protection. These concentrators operate on the principle of rapid pressure swing adsorption (RPSA), whereby, the pressure of the adsorbent beds is cycled at a typical rate of 10 sec/cycle. This rapid cycling improves the oxygen-nitrogen separation efficiency of the concentrator resulting in a significant reduction in the unit's weight and volume. During this cycling the nitrogen component of the air is adsorbed at high pressure and desorbed at low pressure to the surroundings. Concentrators operating on this principle are present onboard the USAF B1-B strategic bomber and the USN AV-8B fighter.
The simplest oxygen concentrator is composed of two cylindrical adsorbent beds containing a zeolite molecular sieve, valving, and an orifice. In a typical two-step cycle, during step 1 of the cycle one bed receives high pressure (20-40 PSIG) feed air which pressurizes the bed and establishes a product oxygen flow, and the nitrogen component of the air is removed by preferential adsorption in the zeolite molecular sieve. Simultaneously, the high pressure gas in the other bed is vented to a lower pressure usually the ambient surroundings, and this depressurization serves to desorb the nitrogen previously adsorbed during the high pressure phase of the cycle. Also, a portion of the product gas from the high pressure bed is fed to the low pressure bed to flush the nitrogen-rich gas from that bed. The orifice serves to control the flow of purge gas. In step 2 of the cycle the adsorbent beds exchange roles. This constant cycling results in a continuous product stream of high purity oxygen.
One limitation of a concentrator containing a zeolite molecular sieve is the maximum oxygen purity of 95% (the remainder is argon). Because the oxygen and argon molecules are similar in size and are nonpolar they both are concentrated upon passage through the beds of zeolite molecular sieve.
U.S. patents of interest include U.S. Pat. No. 4,566,881 to Richter et al which discloses a process and apparatus for producing oxygen with a low fraction of argon from air involving a first adsorption unit comprising at least two adsorbers containing carbon molecular sieve which provides an intermediate product that is enriched with oxygen and depleted of argon by comparison to the supplied N.sub.2 /O.sub.2 /Ar gas mixture. Thereafter the intermediate product is subjected to zeolite adsorption in an adsorption unit. This patent discloses that when the method is carried out with a dry and carbon-dioxide-free air, oxygen is produced with a purity of 99.7 volume percent during the adsorption phase of the zeolite adsorption unit. This patent further discloses that the regeneration of the zeolite-bed adsorbers is interrupted while the first of carbon-bed adsorbers are regenerated by a vacuum pump which is used in common to regenerate the adsorbers. Similarly U.S. Pat. No. 4,190,424 to Armond et al discloses integrating the zeolite and carbon sieve processes to produce oxygen with a purity better than that which can be achieved by either of the known processes operated alone. The overall performance of this process is enhanced by the recycling as feedstock of an oxygen-rich gas stream from the second section to the first. A product stream with a proportion of oxygen as high as 99.7% is cited for one embodiment (see col 3, line 37). In another embodiment, air is provided as feedstock to the zeolite sieve section as also is an oxygen rich gas stream obtained during a feed step in the carbon sieve section. The zeolite sieve section gives a product containing approximately 90% oxygen with 5% nitrogen and 5% argon which is passed as feedstock to the carbon sieve section (see col 3, line 50 et seq.) U.S. Pat. Nos. 4,627,857 and 4,629,476 to Sutt, Jr. are directed to processes for preparation and use of carbon molecular sieves, with a pore size in U.S. Pat. No. 4,627,857 patent of about 3 to about 20 Angstroms, preferably 4 to 10 Angstroms. Other patents relating to oxygen generators or concentrators which rely on molecular sieves include U.S. Pat. Nos. 4,681,602 to Glenn et al, 4,681,099 to Sato et al, 4,661,124 to Hamlin et al, 4,648,888 and 4,561,527 to Rowland, 4,614,525 to Reiss, and 4,272,265 to Snyder; and of these Glenn et al, Hamline et al and Snyder cite aircraft applicability.